Staff photo by Cindy HepnerAgricultural Agent James Johnson, of the Rutgers Cooperative Extension, describes plant hardiness zones and varieties of plants hardy to zone 7a, which encompasses most of Cumberland County.

MILLVILLE — With the recent development of an enhanced Plant Hardiness Zone Map, the United States Department of Agriculture (USDA) has provided farmers and gardeners a more accurate guide to planting in their local climates.

Though the majority of Cumberland County remains in the 7a zone, indicating average annual minimum winter temperatures from 0 to 5 degrees Fahrenheit, the new map offers growers additional knowledge on microclimates, neighboring zones and other factors that could influence plant health and horticultural success.

“The new map is based on temperatures from 1976 to 2005, whereas the old one was from ’74 to ’86, and increased sampling gives you more accurate results,” explained James Johnson, agricultural agent with the Rutgers Cooperative Extension in Rosenhayn.

The 2012 USDA Plant Hardiness Zone Map, available online at www.planthardiness.ars.usda.gov, was developed using National Weather Service data from thousands of stations, feedback from experts in horticultural and geographic specialties and a sophisticated algorithm out of Oregon State University, according to Kim Kaplan, spokeswoman for the Agricultural Research Service, the in-house research agency of the USDA.

The updated map allows users to find their zone by entering a ZIP code or by zooming in to the map as tight as a half-mile area using a Geographic Information System (GIS)-based interactive format. Kaplan explained that the lines on the previous, comparatively rudimentary map, prepared in 1990, were drawn by a graphic artist using a ruler, pen and best judgment to make determinations between reporting stations.

“Now with this algorithm, it weights a bunch of geographic features in that area between the two weather reporting stations, like slope of land, elevation changes, nearness of large bodies of water and prevailing wind, and then says when you weight these factors of the area between these two points, this is what the zone should be,” Kaplan said. “That gives us a precision that we’ve never been able to tackle before. In 1990, you couldn’t have viewed this type of interactive map on a computer, let alone create it.”

With the 2012 map improvements, there now is even a ClimateWise Plant Hardiness app for the iPhone or iPad, which allows users to determine their hardiness zone, as well as the dates of first fall freeze, last spring freeze and growing season length, while out in the garden or at a local nursery considering plant purchases.

The new version of the plant hardiness zone map includes 13 zones representing 10-degree Fahrenheit bands, further divided into 5-degree A and B bands. Zones 12 and 13, representing average low temperatures of 50-60 and 60-70 degrees, respectively, are new to the 2012 map.

Overall, the new map is about one 5-degree Fahrenheit half zone warmer than the previous version, but Kaplan noted that the differences between methodologies make comparing the 1990 map to the 2012 version difficult. The USDA has attributed comparative differences and trends toward warmer zones to data from a longer and more recent time period, stopping short of taking a stand on climate change.

“Of course the question everyone wants to ask is if this is evidence of global warming,” Kaplan acknowledged, “but we try to point out to people that the map simply is not the way to go about proving it. There are much stronger arguments about global warming. A lot of people get very angry because they want this to be their smoking gun, but we try to say it’s the wrong caliber.”

Kaplan added that the 2012 map actually looks more like the hardiness zone map from 1960 than the one from 1990, which she said could be partially explainable by the 1990 map’s sample period, relying heavily on data from “some of the coldest years in the last 100 years” during the ‘70s.

Most of Cumberland County is situated in zone 7a, with a 6b zone encroaching over the county line east of Millville. This area’s climate is warmer than it would be if it weren’t buffered by the ocean and Delaware Bay, Johnson noted.

“We’re in the same zone as central North Carolina; that seems weird since that’s a long way away, but buffering is a big part of it,” Johnson said. “We can sell plants that will grow down into Georgia, and we can also sell plants grown in Massachusetts and New York going north. We’ve got literally thousands of different types of plants that can be grown here.”

“Also, the risk management agency that sets the standards for crop insurance uses the plant hardiness zones to set the nursery crops where they can get insurance for growing nursery crops without protection,” Kaplan added, “so we’ve got to get it right because lawsuits or payoffs may depend on it.”

Noting the example of a variety of coreopsis that had been improperly categorized as hardy to a zone colder than it actually was, leading to the plant being pulled from the market for revision, Kaplan said growers are becoming more sophisticated regarding knowledge of hardiness zones and more demanding of accurate advice from plant breeders.

However, Kaplan emphasized that the map, though accurate, should be used as a guide and is no replacement for experience. She used her own yard, located outside of Washington, D.C., as an example.

“I’m surrounded by one of the largest urban green spaces in the country, so my yard is a half-zone colder than a lot of my friends who live a mile away but are surrounded by blacktop, roads, sidewalks and cement,” Kaplan said. “Really, in each yard, there are different areas for temperatures — the low spot where frost pools first, the warm spot in front of a south-facing white wall — so nothing is going to be more accurate than what gardeners learn in their own yards.”

For seasoned gardeners or farmers who have worked within their own zones and microclimates for years, Kaplan acknowledged that the 2012 hardiness zone map won’t provide much new information or significantly change their plant selections.

Johnson also noted the erratic weather during the past year, with a warm winter followed by very high temperatures in early spring, stimulated plants into growth only to be threatened by unexpected frost and freezes in late spring. A map on temperature averages will do little to help combat this unpredictability, which Johnson said gave growers “an increased number of sleepless nights” this year as they applied protective measures to their plants.

“Where this will make some changes is somebody new moving into the area or just getting into gardening or growing who needs to know about their area, not only what the zone is where they live, but also how close they are to the edge of the zone,” Kaplan said. “If I’m close to the edge of a warmer zone, the chances that I can push my zone are much greater than if I’m sitting next to the edge of a cooler zone.”

Kaplan advised gardeners and farmers with interest to check out the new map and zoom in to their geographical area to assess how close to zone edges their growing area is located. Recognizing proximity to a neighboring climate half a zone warmer or cooler could encourage a grower to try a few different plants, she said.

“But these maps, no matter how accurate they are, are still only a guide,” Kaplan emphasized, “and nothing beats a gardener’s own knowledge of their own garden.”